[Heterogeneity of mammalian antithrombin III]

Affinity chromatography on heparin-Sepharose was used to isolate two forms of antithrombin III(AT) from human, bovine, rabbit and rat blood plasma. The two isolated forms of AT are the major form. AT alpha, making up to 90% of the whole inhibitor molecule, and the minor form, AT beta (10% of AT). The molecular mass of AT beta in all mammalian species under study is by 3-5 kDa lower than that of AT alpha. The isoelectric point for bovine AT alpha lies within the range of 4.95-4.5, whereas that for AT beta--at 5.28-4.76. No significant differences in the progressive antithrombin activity of the major and minor forms of the bovine inhibitor were observed. In contrast, the heparin-cofactor activity of the AT beta-heparin complex exceeds that of the AT alpha-heparin complex--3-fold. The functional differences in the AT forms are due to the differences in their affinities for heparin. It was shown that AT beta exhibits a higher affinity for free and bound heparin.